Injection molding

The previous article mentioned that 90% of flash problems are about the mold not being held shut, not the material itself. But knowing the cause is only the first step—how to fix it is what matters. The following four approaches can cover over 90% of flash cases on the shop floor.

Step One: Clean the Parting Line—Close the Gap

This is the simplest step, and the easiest to overlook. When flash occurs, don't rush to adjust parameters. Open the mold first and shine a flashlight around the parting line. More often than not, a dried pellet of regrind or a tiny speck of rust is sitting there, propping the mold open by a hair when it closes. Gently scrape it off with a copper scraper, wipe the surface clean with a cloth, close the mold, and restart—the flash may be gone.

There's another scenario: leftover flash from the previous cycle pressed onto the parting line. That thin layer acts as a spacer when the mold closes again, and the result is more flash. So cleaning flash isn't just about improving appearance—it's about ensuring the mold can seal properly.

Step Two: Verify Clamping Force—Find the "Phantom Tonnage"

If the parting line is clean and flash persists, it's time to question whether the machine's clamping force reading is accurate.

On toggle-type injection molding machines, hinge wear over time means the number on the screen and the actual force reaching the mold are two different things. If you have access to mold-surface force sensors, take a direct measurement. If not, try a simple test: place thin lead wire or pressure-sensitive paper at the four corners of the mold, clamp and hold pressure, then remove and measure the thickness. If all four corners are consistent and within range, clamping force is evenly distributed. If one side is thicker, that side isn't holding tight.

If clamping force is insufficient, the simplest fix is to increase it—for example, from 500 tons to 550 tons. But don't go overboard; excessive clamping force can damage the mold and tie-bars. If you've maxed out and still get flash, it means this mold is too much for this machine—time to move to a larger-tonnage press.

Step Three: Adjust Process Parameters—Reduce Cavity Pressure

If clamping force checks out, the culprit is likely injection pressure or speed being too high. Plastic rushing into the cavity too fast creates an instantaneous peak pressure that pries the mold open for a split second.

Here's a straightforward move: reduce injection speed by 20% and see if flash improves. If it does, the original speed was indeed too high. Then gradually increase it to find the sweet spot—as slow as possible while still ensuring complete filling and no short shots.

Also check the V-P switchover point (the transition from injection to packing). Many operators habitually set the switchover too late, but the trade-off is that packing pressure gets applied before the cavity is fully filled, essentially using higher pressure to force the mold open. Try moving the switchover point earlier—switch to packing as soon as the cavity is filled, and avoid stacking packing pressure on top of filling pressure.

Melt temperature is worth a look too. The same parameters flash in summer but run fine in winter because ambient heat raises the actual melt temperature. Try lowering the barrel temperature by 5°C to 10°C to reduce flowability, and the flash may stop—provided that filling isn't compromised. If you lower it too much and get short shots, you've traded one problem for another.

Step Four: Inspect the Mold Itself—Is It Flexing Under Pressure?

If you've tried all three steps above and flash persists, it's time to look at the mold itself.

One possibility: insufficient mold rigidity. On deep-cavity, thin-wall parts, the core can elastically deform under high pressure, opening a gap in the middle of the parting line, and material escapes from the center outward. The most effective fix for this is adding support pillars behind the mold—they brace the platen at the points under the most stress, reducing deflection.

Another possibility: parting line wear. Over long production runs, the area near the gate gets eroded by high-pressure melt, creating small craters that prevent a tight seal. This type of wear requires repair—either welding or re-surfacing. It's a mold maintenance issue that needs to be sent to the toolroom. Don't try to tough it out.